The Interactive Effects of Crude Oil and N forms on C Mineralization and Microbial Biomass of a Clay Soil

Toxicity and damages caused by soil pollution with crude oil and its microbial responses may depend on nitrogen bioavailability and its form. So, the main objective of this study is to investigate the effects of crude oil and various forms of N consumption on C mineralization and microbial biomass. The experiment was done in the form of factorial (3×4) design using CRD (completely randomized design) in 3 replicates under laboratorial condition during 126 days. To this intention, clayey soil samples were first contaminated with crude oil levels of 5% and 10% and then, to reduce the toxic effects of crude oil on soil microbial activities, nitrogen salts of KNO3, NH4Cl, and the mixture of them were added. Soil samples were placed at 25±1°C in incubator. C mineralization and microbial biomass were measured at specific intervals. The results indicated that total C mineralization rate was not affected by N application to clayey soil (P>0.05). But the limitation of C caused the microbial activity and consequently total C mineralization to be increased with the addition of crude oil levels. The addition of crude oil and nitrogen simultaneously, did not affect microbial respiration rate, too (P>0.05). All the N treatment and high level of pollutant (crude oil 10%) caused the enhancement of soil microbial biomass C. But in crude oil 5%, an increase of microbial biomass C was shown in the control and NO3 treatments, while this parameter decreased in NH4 and NH4+NO3 treatments. In conclusion, crude oil stimulates microbial activity and biomass by reason of containing C and the other nutrients and also biodegradability of some of its compounds and derivatives. Its toxic elements and compounds have inhibitory effects on microbial communities. However, N addition to crude oil contaminated soils may cause the reduction of harmful effects of toxic hydrocarbon compounds.